Page 167 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Transport Processes at the Nephron the proximal to the distal segments of the
tubules.
Filtration of solutes. The glomerular filtrate Whereas permeability of the two mem-
also contains small dissolved molecules of branes in series is decisive for transcellular
plasma (ultrafiltrate) (! p. 152). The glomeru- transport (reabsorption, secretion), the tight-
lar sieving coefficient GSC of a substance (= con- ness of tight junctions (! p. 18) determines the
centration in filtrate/concentration in plasma paracellular permeability of the epithelium for
water) is a measure of the permeability of the water and solutes that cross the epithelium by
glomerular filter for this substance (! p. 148). paracellular transport. The tight junctions in
Kidneys, Salt, and Water Balance a radius of r " 4.4 nm (molecular mass makes the epithelium well equipped for para-
the proximal tubule are relatively permeable
Molecules with a radius of r ! 1.8 nm (molecu-
to water and small ions which, together with
lar mass ! ca. 10 000 Da) can freely pass
through the filter (GSC ! 1.0), while those with
the large surface area of the cell membranes,
and transcellular mass transport (! D, column
" 80 000 DA, e.g., globulins) normally cannot
2). The thin limbs of Henle’s loop are relatively
pass through it (GSC = 0). Only a portion of
molecules where 1.8 nm ! r ! 4.4 nm applies
“leaky”, while the thick ascending limb and the
rest of the tubule and collecting duct are
are able to pass through the filter (GSC ranges
“moderately tight” epithelia. The tighter
between 1 and 0). Negatively charged particles
(e.g., albumin: r = 3.4 nm; GSC ! 0.0003) are
epithelia can develop much higher trans-
equal radius because negative charges on the
“leaky” epithelia.
Measurement of reabsorption, secretion
wall of the glomerular filter repel the ions.
7 less permeable than neutral substances of epithelial chemical and electrical gradients than
and excretion. Whether and to which degree a
When small molecules are bound to plasma
proteins (protein binding), the bound fraction substance filtered by the glomerulus is reab-
is practically non-filterable (! p. 24). sorbed or secreted at the tubule and collecting
duct cannot be determined based on its uri-
Molecules entrapped in the glomerular filter are
believed to be eliminated by phagocytic mesangial nary concentration alone as concentrations
macrophages (! p. 94ff.) and glomerular podo- rise due to the reabsorption of water
cytes. (! p. 164). The urinary/plasma inulin (or
creatinine) concentration ratio, U in/P in is a
Tubular epithelium. The epithelial cells lining measure of the degree of water reabsorption.
the renal tubule and collecting duct are polar These substances can be used as indicators be-
cells. As such, their luminal (or apical) mem- cause they are neither reabsorbed nor secreted
brane on the urine side differs significantly
from that of the basolateral membrane on the (! p. 152). Thus, changes in indicator concen-
tration along the length of the tubule occur
blood side. The luminal membrane of the prox- due to the H 2O reabsorption alone (! A). If
imal tubule has a high brush border consisting U in/P in = 200, the inulin concentration in the
of microvilli that greatly increase the surface final urine is 200 times higher than in the orig-
area (especially in the convoluted proximal inal filtrate. This implies that fractional excre-
tubule). The basolateral membrane of this tion of H 2O (FE H 2 O) is 1/200 or 0.005 or 0.5% of
tubule segment has deep folds (basal laby- the GFR. Determination of the concentration of
rinth) that are in close contact with the intra- a (freely filterable and perhaps additionally
cellular mitochondria (! p. 9 B), which pro- secreted) substance X in the same plasma and
duce the ATP needed for Na -K -ATPase (! p. urine samples for which U in/P in was measured
+
+
26) located in the basolateral membrane (of all will yield U x/P x. Considering U in/P in, the frac-
epithelial cells). The large surface areas (about tional excretion of X, FE X can be calculated as
100 m ) of the proximal tubule cells of both follows (! A and D, in % in column 5):
2
kidneys are needed to reabsorb the lion’s share FE X # (U X/P X)/(U In/P In) [7.9]
of filtered solutes within the contact time of a
.
couple of seconds. Postproximal tubule cells Eq. 7.9 can also be derived from C x/C in
(! p. 152) when simplified for V U. The frac-
154 do not need a brush border since the amount of tional reabsorption of X (FR X) is calculated as
substances reabsorbed decreases sharply from
FR X # 1 – FE X [7.10]
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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